JP3679179B2 - Steel pipe with excellent earthquake resistance - Google Patents

Description

【００２６】
【表２】

【００２７】
表２に示すように、形状に応じた第２相の面積分率が本発明の範囲を満足する鋼管Ａ−１、Ａ−２、Ａ−４、Ｂ−１、Ｃ−１、Ｄ−１はいずれも応力歪曲線の上記領域での勾配が正となり、５％以上の一様伸びと８０％以下の降伏比を有していた。
【００２８】
一方、ｔ／Ｄが０．０２未満で第２相組織の面積分率が２０％未満の鋼管Ａ−３、Ｂ−２、Ｄ−２では勾配が一部でゼロとなり、さらにＤ−２では降伏比も８０％を超えていた。また、面積分率が８０％を超えている鋼管Ａ−５では充分な一様伸びが得らなかった。
【００２９】
【発明の効果】
以上説明したように、本発明によれば、大地震の際に軸方向に作用する引張・圧縮応力に対して、大径薄肉でも局部座屈を起こしにくく、脆性的な破断が発生し難い耐震性に優れた鋼管を得ることができる。したがって、本発明の鋼管を用いることにより、大地震が発生した際に、ガスパイプラインや水道配管の破損と内部流体の流出を防止することができる。
【図面の簡単な説明】
【図１】 実管圧縮試験に用いた試験機および試験体を説明するための図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for manufacturing an earthquake-resistant steel pipe having excellent local buckling resistance at the time of an earthquake, which is used for gas pipelines, water pipes and the like.
[0002]
[Prior art]
Carbon steel pipes such as UOE steel pipes, spiral steel pipes, seamless steel pipes, ERW steel pipes, press-bend steel pipes, and low-alloy steel pipes can be manufactured in large quantities and stably, which is combined with their excellent economic efficiency and weldability. Therefore, it is widely used as a transportation pipe for fluids such as gas pipelines and water pipes.
[0003]
However, when a large earthquake occurs, large tensile and compressive forces are repeatedly applied in the longitudinal direction of these steel pipes, and local buckling occurs in steel pipes with a large outer diameter / pipe thickness ratio. May lead to cracking or fracture.
[0004]
Conventional steel pipes for construction include, for example, JP-A-3-173719, JP-A-5-65535, JP-A-5-117746, JP-A-5-117747, JP-A-5-156357, JP-A-6-49540. No. 6, JP-A-6-49541, JP-A-6-128641, JP-A-6-264143, and JP-A-6-264144, the yield stress and the tensile strength are The yield ratio, which is a small ratio, has been proposed, but these all relate to the ability to absorb plastic deformation against the bending stress of the column, and are due to local buckling against the axial force of compression and tension after the occurrence of local buckling. There have been no studies to prevent the occurrence of brittle cracks.
[0005]
In addition, in line pipes for transporting fluids such as gas, resistance to internal pressure, which acts in the circumferential direction such as ductile fracture and brittle fracture, has been studied, but bending deformation during laying against axial external forces Except for, it is hardly considered.
[0006]
[Problems to be solved by the invention]
The present invention has been made in view of such circumstances, and with respect to tensile / compressive stress acting in the axial direction in the event of a large earthquake, local buckling is unlikely to occur even with a large diameter and thin wall, and brittle fracture occurs. It is difficult to provide a steel pipe excellent in earthquake resistance suitable for gas pipelines, water pipes and the like.
[0007]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present invention includes, by weight, C: 0.05 to 0.25%, Mn: 0.5 to 2.0%, and Cu: 0.05 to 0.50. %, Ni: 0.05 to 0.50%, Cr: 0.05 to 0.50%, Mo: 0.05 to 0.50%, Nb: 0.005 to 0.10%, V: 0.00. 005~0.10%, Ti: 0.005~0.10% of one or more, Ri Do the balance Fe and unavoidable impurities, when the ratio of the tube thickness and the outer diameter is less than 0.02 The area fraction of the second phase structure consisting of at least one of pearlite, bainite, martensite in the microstructure is 20% or more and 30% or less, and the remainder has a ferrite structure as the first phase, tube thickness and when the ratio between the outer diameter of 0.02 or more, in the second phase structure area fraction of 25% at least 10% less, residual There a steel pipe having a ferrite structure as a first phase, comprising 80% or less of yield ratio, and tensile using a tensile test pieces taken in the specimen longitudinal direction is aligned with the axial direction of the steel pipe test In the obtained nominal stress-nominal strain curve, the gradient of nominal stress-nominal strain is always positive in the strain range of 0% to 5%. Provided is a steel pipe excellent in compression buckling resistance used for a line pipe subjected to stress.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
In order to facilitate understanding of the present invention, first, common technical knowledge of those skilled in the art related to the present invention will be described. Buckling resistance is work hardening coefficient of the steel tube (n value; true stress (sigma _A) - is represented by n in the case of representing the relationship between true strain (e) in sigma _A = Ce ⁿ⁾ that there is a correlation, It is known that a steel pipe having a high work hardening index (n value) has a high critical buckling strain (has buckling resistance). Further, there is a clear correlation between the work hardening index (n value) and the yield ratio, and it is known that the higher the work hardening index (n value), the lower the yield ratio. That is, since the slope of the stress-strain curve increases as the work hardening index increases, it can be said that a material with a large work hardening index has a large increase in strength due to deformation. Since the material having a higher work hardening index has a higher strength increase after hardening, the yield ratio represented by the ratio between the tensile strength and the yield strength becomes smaller. On the other hand, the occurrence of buckling of the steel pipe is caused by the deformation concentrated on the weakest part of the steel pipe due to the heterogeneity of the material and shape. For this reason, the work hardening characteristics (= yield ratio) of steel pipe materials are closely related to the buckling behavior of steel pipe materials. In order to obtain steel pipes with excellent buckling resistance, materials with low yield ratio (= work hardening index Large material) must be used.
Next, there are two types of stress-strain curves: the case where yield occurs due to uniform deformation (the yield point is not clear) and the case where yield occurs due to non-uniform deformation (the yield point appears clearly). is there. And since the occurrence of buckling of the steel pipe is due to the inhomogeneity of the material and shape, the buckling characteristics are low when yielding occurs due to non-uniform deformation. On the other hand, when yielding occurs due to uniform deformation, the buckling characteristics are excellent.
In order to evaluate the buckling resistance against the compressive force acting in the axial direction of the steel pipe, the present inventors used a test machine and a test body shown in FIG. Tests and various material investigation tests were conducted to investigate the correlation between the material characteristics of steel pipes and the behavior of local buckling. As a result, it was found that the presence or absence of local buckling has the following correlation with the tensile properties in the axial direction of the steel pipe. That is, a tensile test was performed using a tensile test piece collected with the test piece longitudinal direction coinciding with the axial direction of the steel pipe, and in the obtained nominal stress-nominal strain curve, the on-load strain amount was 5% from the yield point. For any of these strains, the steel pipe with a positive nominal stress / nominal strain gradient has a significantly larger limit of outer diameter / pipe thickness that causes local buckling than a steel pipe with a zero or negative gradient. Less likely to cause local buckling.
[0009]
Next, after plastic deformation due to compressive stress, the correlation between the occurrence of brittle cracks and fractures when subjected to tensile stress and the material of the steel pipe was investigated. As a result, even in a steel pipe plastically deformed due to compressive stress, in a tensile test of a specimen taken by aligning the longitudinal direction of the specimen with the axial direction of the steel pipe, if the uniform elongation is 5% or more, a brittle crack or It was found that no breakage occurred. Furthermore, the ratio of the yield strength to the tensile strength in the tensile test, and the smaller the yield ratio, the greater the resistance to fracture.
[0010]
As a result of investigating the relationship between the nominal stress-nominal strain curve having the above-described characteristics and the shape and microstructure of the steel pipe based on such results, the ratio t / For steel pipes with D less than 0.02, the area fraction of the second phase structure composed of at least one of pearlite, bainite, and martensite is 20% to 80%, and the t / D is 0.02 or more. Then, when the area fraction of the 2nd phase structure became 10% or more and 80% or less, it discovered that the above nominal stress-nominal strain curves could be obtained.
[0011]
Based on such knowledge, in the present invention, when t / D is less than 0.02, the area fraction of the second phase structure composed of at least one of pearlite, bainite, and martensite in the microstructure is 20. % Or more and 80% or less, and when t / D is 0.02 or more, the area fraction of the second phase structure is 10% or more and 80% or less, and the balance is ferrite as the first phase. To provide steel pipes with excellent earthquake resistance.
Summarizing the above, the present invention provides a steel material having a predetermined chemical composition. By defining the second phase fraction and t / D when forming into a steel pipe within a predetermined range, a yield of 80% or less is obtained. Qualitative knowledge that the stress-strain curve can yield by uniform deformation (the nominal stress-nominal strain gradient is always positive in the 0% to 5% strain range) Based on the above, the range is quantified. When the ratio of the tube thickness to the outer diameter (t / D) is less than 0.02, the area fraction of the second phase structure is 20% or more and 30% or less. When the ratio (t / D) between the tube thickness and the outer diameter is 0.02 or more, the area fraction of the second phase structure is 10% or more and 25% or less. It has a yield ratio, and yields a stress-strain curve with uniform deformation. -It is intended to provide a steel pipe excellent in compression buckling resistance used for a line pipe on which compressive stress acts.
[0012]
Here, when t / D is less than 0.02, the area fraction of the second phase structure is less than 20%, and when t / D is 0.02 or more, the area fraction of the second phase structure is 10 If it is less than%, there is a case where the slope of the stress-strain curve does not become positive. On the other hand, if it exceeds 80%, sufficient uniform elongation and a yield ratio of 80% or less cannot be obtained. Therefore, when the area fraction of the second phase structure is 20% or more and 80% or less and t / D is 0.02 or more, the area fraction of the second phase structure is 10% or more and 80% or less. To do.
[0013]
In the present invention, the manufacturing method of the steel pipe is not limited, and any of UOE steel pipe, spiral steel pipe, seamless steel pipe, ERW steel pipe, press bend steel pipe, and the like can be used as long as it satisfies the predetermined characteristics. Good.
[0014]
The chemical composition of steel is not particularly limited, but the following composition is preferable.
[0015]
That is, by weight, C: 0.05 to 0.25%, Mn: 0.5 to 2.0%, and if necessary, Cu: 0.05 to 0.50%, Ni: 0.0. 05-0.50%, Cr: 0.05-0.50%, Mo: 0.05-0.50%, Nb: 0.005-0.10%, V: 0.005-0.10% , Ti: One containing 0.005 to 0.10% or two or more is preferable. The reason why the steel having such a component range is preferable is as follows.
[0016]
C: 0.05 to 0.25% Steel having a carbon content outside this range has an increased possibility of weld cracking when welded, and cannot be welded after forming a steel pipe. Therefore, the C content is preferably in the range of 0.05 to 0.25%.
[0017]
Mn: 0.5 to 2.0% Mn is an element effective for obtaining sufficient strength and toughness as a structural steel, but if it is less than 0.5%, the effect is small, and it exceeds 2.0%. In addition, the toughness of the base metal and the welded part and the weldability are deteriorated. Therefore, the amount of Mn is preferably in the range of 0.5 to 2.0%.
[0018]
Cu: 0.05 to 0.50% Ni: 0.05 to 0.50% Cr: 0.05 to 0.50% Mo: 0.05 to 0.50% Cu, Ni, Cr, and Mo are strong. Although it is an element effective for increasing, if less than 0.05%, the effect is not exhibited effectively, and if it exceeds 0.50%, the toughness and weldability of the base metal weld of the steel plate are deteriorated. Accordingly, these amounts are each preferably in the range of 0.05 to 0.50%.
[0019]
Nb: 0.005 to 0.10% Nb is an element effective for improving the toughness and strength of the steel sheet. However, if the amount is less than 0.005%, the effect cannot be exhibited effectively. If it exceeds 10%, the toughness of the weld is deteriorated. Therefore, the Nb content is preferably in the range of 0.005 to 0.10%.
[0020]
V: 0.005 to 0.10% V is an element effective for increasing the strength of the steel sheet. However, if the amount is less than 0.005%, the effect cannot be exhibited effectively. If it exceeds 50%, the toughness of the welded portion deteriorates. Therefore, the V amount is preferably in the range of 0.005 to 0.10%.
[0021]
Ti: 0.005 to 0.10% Ti is an element effective for improving the toughness of a steel sheet and preventing slab damage during casting. However, when its amount is less than 0.005%, the effect should be exhibited effectively. However, if it exceeds 0.10%, the toughness of the welded portion is deteriorated. Therefore, the Ti amount is preferably in the range of 0.005 to 0.10%.
[0022]
Further, the production conditions are not particularly limited as long as the above structure can be obtained. For example, for steel sheets having the above-described composition, the rolling conditions are controlled, or the steel pipes during or after pipe forming are heat treated. By applying the processing treatment, for example, after the end of rolling, the accelerated cooling is started from the range of Ar3 temperature to Ar3-80 ° C determined by the chemical composition of the steel, and the accelerated cooling is stopped at 400 ° C or higher. You can get an organization.
[0023]
【Example】
Hereinafter, specific examples of the present invention will be described.
[0024]
A steel having the chemical composition shown in Table 1 (A is a reference steel type, B, C, and D are steel types of the present invention) is hot-rolled to form a steel plate, then formed and welded at the ends to obtain a UOE steel pipe. It was. Steel pipes A-1, A-2, A-3, A-4, A-5, and C-1 change the area ratio of the second phase structure by changing the accelerated cooling start temperature after rolling to 700 to 760 ° C. Changed. The steel pipes B-1, B-2, D-1, and D-2 are cooled at a cooling rate of 10 ° C./second after being heated from 800 ° C. to 850 ° C. after steel pipe forming welding. Table 2 shows the shape of the steel pipe thus manufactured and the area fraction of at least one second phase structure of pearlite, bainite, and martensite, gradient up to 5% on-load strain, uniform elongation, yield. Indicates the ratio. In the gradient column of Table 2, + indicates that the gradient is always positive in the above region, and X indicates that the gradient is negative or zero in this region.
[0025]
[Table 1]

[0026]
[Table 2]

[0027]
As shown in Table 2, the steel pipes A-1, A-2, A-4, B-1, C-1, D-1 whose area fraction of the second phase according to the shape satisfies the scope of the present invention. Each had a positive slope in the above region of the stress-strain curve, and had a uniform elongation of 5% or more and a yield ratio of 80% or less.
[0028]
On the other hand, in steel pipes A-3, B-2, and D-2 where the t / D is less than 0.02 and the area fraction of the second phase structure is less than 20%, the gradient is partially reduced to zero, and further in D-2 The yield ratio also exceeded 80%. Moreover, sufficient uniform elongation was not obtained in steel pipe A-5 whose area fraction exceeded 80%.
[0029]
【The invention's effect】
As described above, according to the present invention, with respect to tensile / compressive stress acting in the axial direction in the event of a large earthquake, even if it has a large diameter and thin wall, local buckling is unlikely to occur, and brittle fracture is unlikely to occur. A steel pipe with excellent properties can be obtained. Therefore, by using the steel pipe of the present invention, it is possible to prevent the gas pipeline and the water pipe from being damaged and the internal fluid from flowing out when a large earthquake occurs.
[Brief description of the drawings]
FIG. 1 is a view for explaining a testing machine and a test body used in an actual pipe compression test.

Claims (1)

In wt%, C: 0.05 to 0.25%, Mn: 0.5 to 2.0%, and Cu: 0.05 to 0.50%, Ni: 0.05 to 0.50% , Cr: 0.05 to 0.50%, Mo: 0.05 to 0.50%, Nb: 0.005 to 0.10%, V: 0.005 to 0.10%, Ti: 0.005 0.10% of one or more, Ri Do the balance Fe and unavoidable impurities, when the ratio between the tube thickness and the outer diameter is less than 0.02, pearlite in the microstructure, bainite, martensite The area fraction of the second phase structure consisting of at least one of the sites is 20% or more and 30% or less, and the balance has the ferrite structure as the first phase, and the ratio of the tube thickness to the outer diameter is 0.02. above the case, the second phase structure area fraction of 10% or more and 25% or less, and the balance have a ferritic structure as the first phase That a steel pipe having a 80% or less of yield ratio, and the test piece longitudinal direction is aligned with the axial direction of the steel pipe subjected to a tensile test using a tensile test piece was collected, resulting nominal stress - In nominal strain curves, nominal stress-nominal strain gradient is always positive in the strain range of 0% to 5% , used for line pipes with tensile and compressive stress acting in the axial direction during earthquakes Steel tube with excellent local buckling resistance.

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